Chitosan-enhanced nonswelling hydrogel with stable mechanical properties for long-lasting underwater sensing.

Existing anti-swelling hydrogels with poor mechanical strength restrict their underwater human monitoring as wearable electronic sensing equipment. Herein, a nonswelling double network (DN) hydrogel with strong self-recoverability (97.22%) was developed by adding chitosan (CS) to poly(acrylic acid-2-methoxyethyl acrylate)-Fe3+ [P(AA-MEA)-Fe] network. Owing to the introduction of CS, the hydrogel displayed excellent nonswelling properties under aqueous solutions (pH = 1, 4 and 7), physiological saline, seawater, dodecane, n-hexane and chloroform. Besides, CS improved mechanical properties of hydrogel through non-covalent network (large stretchability of 1199%, tensile strength of 0.462 MPa and toughness of 2.01 MJ/m3). Surprisingly, the hydrogel still reached the extensibility (1072%) and tensile stress (0.467 MPa) even after immersing in water for 7 days. Fabricating hydrogel as flexible strain sensor, periodic real-time signals of human movements (e.g., joint actions and electronic skin touching) were accurately monitored under the water and seawater. The nonswelling P(AA-MEA)-CS-Fe hydrogel shows huge potential in underwater sensing.